In anticipation of the holidays I'm trying to decide my next purchase for my DSLR. Extra spending money is tight, so it will have to be one or the other. I have a Canon T1i with a 18mm-55m IS and a 55mm-250mm IS lens.

My goal is to be able to take some good indoor pictures during the holidays (kids opening presents, etc). Which would benefit me the most for indoor shooting, a Yongnuo YN-468 E-TTL external flash or a Canon EF 50mm f/1.8 II lens? Both can be had for about $100 each.